Method for producing scratch resistant coatings, especially for producing multi-layer enamels

ABSTRACT

A process for producing scratch-resistant coatings, especially scratch-resistant multicoat finishes, involving use of coating compositions which after curing have a storage modulus E′ in the rubber-elastic range of at least 10 7.5  Pa and a loss factor tan δ at 20° C. of at least 0.05, the storage modulus E′ and the loss factor having been measured by dynamic mechanical thermoanalysis on free films having a film thickness of 40±10 μm, and comprise as binders one or more polyacrylate resins having a hydroxyl number of from 100 to 240, an acid number of from 0 to 35, and a number-average molecular weight of from 1500 to 10,000, and as crosslinkers one or more free or blocked isocyanates and/or triazine-based components which crosslink with the hydroxyl groups of the binder to form ether and/or ester structures.

CROSS-REFERENCE TO RELATED APPLICATION

The present application is a divisional application of U.S. Ser. No.09/380,524, filed on Nov. 8, 1999, which is a 371 of PCT/EP98/01265,filed on Mar. 6, 1998, which is incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to a process for producingscratch-resistant coatings, especially scratch-resistant multicoatfinishes.

The present invention relates, furthermore, to coating compositionssuitable for this process.

BACKGROUND ART

In past years, great progress has been made in developing acid-resistantand etch-resistant clearcoats for the OEM finishing of automobiles. Morerecently, there is now an increasing desire in the automotive industryfor scratch-resistant clearcoats which at the same time retain the levelattained hitherto in terms of their other properties.

At present, however, there are different test methods for thequantitative assessment of the scratch resistance of a coating, examplesbeing testing by means of the BASF brush test, by means of the washingbrush unit from the company AMTEC, or various test methods of automakersand others. A disadvantage, however, is that it is frequently impossibleto correlate the individual test results; In other words, the testresults for one and the same coating may have very different outcomesdepending on the test method chosen, and passing one scratch resistancetest does not, under certain circumstances, permit conclusions to bedrawn about the behavior of that coating in a different scratch test.

There is therefore a desire for a standardized method of quantitativelyassessing the scratch resistance which enables reliable statements to bemade about the scratch resistance of the coating from just oneexamination of the sample. In particular, the result of this examinationshould permit reliable conclusions to be drawn about the scratchresistance of the coating in the various abovementioned scratchresistance tests.

The literature, indeed, has already described a number of investigationsrelating to the physical processes taking place during the production ofscratches in a coating, and correlations, derived therefrom, between thescratch resistance and other physical parameters of the coating. Acontemporary review of various literature relating to scratch-resistantcoatings can be found, for example, in J. L. Courter, 23^(rd) AnnualInternational Waterborne, High-Solids and Powder Coatings Symposium, NewOrleans 1996.

Furthermore, for example, the article by S. Sano et al., “Relationshipbetween Viscoelastic Property and Scratch Resistance of Top-Coat ClearFilm”, Toso Kagaku 1994, 29 (12), pages 475-480 uses a washing brushtest to determine the scratch resistance of different, heat-curingmelamine/acrylate or isocyanate/acrylate systems and correlates thescratch resistance found with viscoelastic properties of the coating.

From the test results described in that article, the authors concludethat coatings would exhibit good scratch resistance when either theso-called “inter-crosslinking molecular weight” was below 500 or whenthe glass transition temperature was 15° C. or less, it being necessary,however, in the case of clearcoat films in the automotive sector, forthe glass transition temperature to be above 15° C. in order to achievesufficient hardness of the coatings.

In the article by M. Rösler, E. Klinke and G. Kunz in Farbe+Lack, Volume10, 1994, pages 837-843, too, the scratch resistance of various coatingsis investigated by means of different test methods. The article foundthat, under a given load, hard coatings exhibit greater damage and thuslower scratch resistance than soft coatings.

Still further, in the conference report of B. V. Gregorovich and P. J.McGonical, Proceedings of the Advanced Coatings Technology Conference,Illinois, USA, Nov. 3-5, 1992, pages 121-125, it is found thatincreasing the plastic nature (toughness) of coatings improves thescratch resistance, owing to the improvement in plastic flow (greaterhealing), although limits are imposed on the increase in plastic natureby the other properties of the coating.

Further yet, P. Betz and A. Bartelt in Progress in Organic Coatings, 22(1993), pages 27-37 disclose various methods of determining the scratchresistance of coatings. That article makes reference, furthermore, tothe fact that the scratch resistance of coatings is influenced not onlyby the glass transition temperature but also, for example, by thehomogeneity of the network.

That article proposes increasing the scratch resistance of clearcoatcoatings by incorporating siloxane macromonomers, since these siloxanemacromonomers lead to increased homogeneity of the clearcoat surfaceand, above 60° C., to an improved plastic flow.

The correlation between storage modulus and crosslinking density,furthermore, is known, for example, from Loren W. Hill, Journal ofCoatings Technology, Vol. 64, No. 808, May 1992, pages 29 to 41.However, that article contains no statements or indications as to howscratch-resistant coatings can be obtained.

DE-C-39 18 968, furthermore, discloses a process for coating surfacesusing clearcoats, based on hydroxyl-containing resins andpolyisocyanates, whose composition is established such that theclearcoat film, after curing, has a molecular weight of the chainbetween the crosslinks of up to 200 (measured in accordance with thexylene swelling method). However, even these clearcoats are still inneed of improvement in respect of the scratch resistance of theresultant coatings.

Finally, DE-A-43 10 414 and DE-A-42 04 518 disclose nonaqueousclearcoats based on hydroxyl-containing acrylate resins and isocyanates,for the production of multicoat finishes, where the resulting coatingsare notable for improved scratch resistance and good other serviceproperties. However, even with these clearcoats there is a desire for aneven greater improvement in scratch resistance.

Although many scratch-resistant finishes and method of producing sameare known, a need still exists in the art for a process for producingcoatings having further-improved scratch resistance. At the same time,the coating compositions employed in this process should, furthermore,be suitable as a clearcoat and/or topcoat for producing a multicoatfinish, especially in the automotive sector. In addition, the coatingsshould exhibit high gloss, good chemical resistance and good weatheringstability.

SUMMARY OF THE INVENTION

The object of the present invention is, therefore, to provide suchcoating compositions and a process for producing coating using suchcoating compositions which fulfills the need in the art.

It is another object of the present invention to establish a criterionfor assessing the scratch resistance of the cured coating objectively,independently of the particular test method chosen, on the basis ofphysical parameters. The method of determining the physical parametersaccording to the invention is able to be used under practical conditionsand with sufficient accuracy to enable the scratch resistance to becharacterized in a way which is at least adequate to visual evaluation.

These object are, surprisingly, achieved by a process for producingscratch-resistant coatings which comprises the steps of providing atleast one coating composition and forming a scratch-resistant coating ona surface using the coating compositions wherein the coatingcompositions,

after curing, have a storage modulus E′ in a rubber-elastic range of atleast 10^(7.5) Pa and a loss factor tan δ at 20° C. of at least 0.05,the storage modulus E′ and the loss factor tan δ having been measured bydynamic mechanical thermoanalysis on homogeneous free films having afilm thickness of 40±10 μm the coating compositions comprise

as binders one or more polyacrylate resins having a hydroxyl number offrom 100 to 240, preferably more than 160 to 220 and, with particularpreference, from 170 to 200, an acid number of from 0 to 35, preferablyfrom 0 to 25, and a number-average molecular weight of from 1500 to10,000, preferably from 2500 to 5000, and as crosslinkers one or morefree isocyanates, blocked isocyanates and triazine-based componentswhich crosslink with hydroxyl groups of the binders to form ether and/orester structures.

The present specification relates, furthermore, to a process forproducing a scratch-resistant multicoat finish and to coatingcompositions suitable for this process.

It is surprising and was not foreseeable that, merely by measuring theviscoelastic properties of free films by means of dynamic mechanicalthermoanalysis (also referred to for short below as DMTA) there isavailable a universal, representative selection criterion for theprovision of coating compositions which lead to scratch-resistantcoatings. At the same time, the results of the DMTA measurements can becorrelated with the results of the different test methods for scratchresistance, so that, on the basis solely of the results of the DMTAmeasurements, statements are possible about the results in other scratchresistance tests, such as, for example, the BASF brush test or the AMTECtest, or various test methods of the automakers.

Other objects, advantages and salient features of the invention will beapparent from the following detailed description.

DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS OF THEINVENTION

In the text below, the coating compositions employed in the process ofthe invention for producing scratch-resistant coatings are elucidatedfirst of all.

It is essential to the invention that the coating composition beselected such that the cured coating composition has a storage modulusE′ in the rubber-elastic range of at least 10^(7.5) Pa, preferably of atleast 10^(7.6) Pa and, with particular preference, of at least 10^(7.7)Pa and a loss factor at 20° C. of at least 0.05, preferably at least0.07, the storage modulus E′ and the loss factor tan δ having beenmeasured by dynamic mechanical thermoanalysis on homogeneous free filmshaving a film thickness of 40±10 μm. The loss factor tan δ is defined asthe quotient between the loss modulus E″ and the storage modulus E′.

Dynamic mechanical thermoanalysis is a widely known measurement methodfor determining the viscoelastic properties of coatings and isdescribed, for example, in Murayama, T., Dynamic Mechanical Analysis ofPolymeric Material, Elsevier, N.Y., 1978 and Loren W. Hill, Journal ofCoatings Technology, Vol. 64, No. 808, May 1992, pages 31 to 33.

The measurements can be carried out using, for example, the instrumentsMK II, MK III or MK IV from the company Rheometric Scientific.

The storage modulus E′ and the loss factor tan δ are measured onhomogeneous free films. The free films are prepared in conventionalmanner by applying the coating composition to, and curing it on,substrates to which the coating composition does not adhere. Examples ofsuitable substrates that may be mentioned are glass, Teflon® and, inparticular, polypropylene. Polypropylene has the advantage of readyavailability and is therefore normally employed as a support material.

The film thickness of the free films employed for the measurement isgenerally 40±10 μm.

The specific selection of the coating compositions by way of the valueof the storage modulus in the rubber-elastic range and of the lossfactor at 20° C. of the cured coating compositions simplifies theprovision of coatings having the desired good scratch resistance, sinceboth parameters can be determined by means of simple DMTA measurements.

In this context it is surprising that even coating materials which haveonly a moderate or even low plastic component but yet have a high tovery high storage modulus give rise to coatings having high scratchresistance. As the tan δ value increases, and with a sufficiently highstorage modulus E′, however, there is generally an increase in thescratch resistance of the resulting coatings. At the same time, however,other performance properties of the coatings may deteriorate, so that inthis case, owing to the other properties, the tan δ value should be notmore than 0.2, preferably not more than 0.1.

The coating compositions of the invention that are employed in theprocess for producing scratch-resistant coatings comprise as binders oneor more polyacrylate resins having a hydroxyl number of from 100 to 240,preferably more than 160 to 220 and, with particular preference, from170 to 200, an acid number of from 0 to 35, preferably from 0 to 25, anda number-average molecular weight of from 1500 to 10,000, preferablyfrom 2500 to 5000. Suitable polyacrylate resins in this case are, inprinciple, all those having the stated parameters (OH number, acidnumber and molecular weight) provided they lead after crosslinking tocoatings having the stated viscoelastic parameters.

As is generally known, however, the particular chosen monomercomposition, for example, also has an influence, inter alia, on theseviscoelastic parameters of the cured coating. For example, the storagemodulus E′ generally decreases as the styrene content of the acrylateresins goes up. It is therefore preferred that the binders employedcomprise acrylate resins containing (in copolymerized form) not morethan 15% by weight, based on the overall weight of all monomers of theacrylate resin, of vinylaromatic hydrocarbons, especially styrene.

Furthermore, as binders in the coating compositions of the invention,preference is given to the use of acrylate resins having as many primaryhydroxyl groups as possible (with particular preference, at least 50%and up to 100% of the OH groups are primary OH groups), since a morecomplete reaction of the primary OH groups, in comparison to secondaryOH groups, likewise enables an increase in the storage modulus E′ of thecured coatings.

In addition, preference is given to the use as binders of acrylateresins having a glass transition temperature of not more than 70° C.,and particular preference to those having a glass transition temperatureof from −40 to +30° C.

The glass transition temperature can be calculated approximately by theskilled worker with the aid of the formula

1/Tg=ΣW _(n) /T _(gn)

Tg=Glass transition temperature of the polymer

Wn=Weight fraction of the nth monomer

Tgn=Glass transition temperature of the homopolymer of the nth monomer.

Finally, other binders employed are preferably acrylate resinsobtainable by polymerizing

(a) from 25 to 62, preferably from 41 to 57% by weight of4-hydroxy-n-butyl acrylate or 4-hydroxy-n-butyl methacrylate or amixture of 4-hydroxy-n-butyl acrylate and 4-hydroxy-n-butylmethacrylate,

(b) from 0 to 36% by weight, preferably from 0 to 20% by weight, of anon-(a) hydroxyl-containing ester of acrylic acid or ahydroxyl-containing ester of methacrylic acid or a mixture of suchmonomers,

(c) from 28 to 75% by weight, preferably from 34 to 54% by weight, of anon-(a) and non-(b) aliphatic or cycloaliphatic ester of methacrylicacid having at least 4 carbon atoms in the alcohol residue or a mixtureof such monomers,

(d) from 0 to 3, preferably from 0 to 2% by weight of an ethylenicallyunsaturated carboxylic acid or a mixture of ethylenically unsaturatedcarboxylic acids, and

(e) from 0 to 20, preferably from 5 to 15% by weight of a non-(a),non-(b), non-(c) and non-(d) ethylenically unsaturated monomer or amixture of such monomers

to give the polyacrylate resin, the sum of the weight fractions ofcomponents (a), (b), (c), (d) and (e) always being 100% by weight andthe composition of component (c) being chosen such that polymerizationof component (c) alone gives a polymethacrylate resin having a glasstransition temperature of from 0 to +80, preferably from 0 to +60° C.

The polyacrylate resins which are employed with preference in accordancewith the invention can be prepared by polymerization techniques whichare common knowledge and have been described on numerous occasions (cf.e.g.: Houben-Weyl, Methoden der organischen Chemie, 4^(th) Edition,Volume 14/1, pages 24 to 255 (1961)), and the disclosure of thispublication is incorporated herein by reference.

The polyacrylate resins which are employed with preference in accordancewith the invention are prepared in particular with the aid of thesolution polymerization technique. This technique customarily comprisesintroducing an organic solvent or solvent mixture and heating it toboiling. The monomer mixture to be polymerized along with one or morepolymerization initiators are then added continuously to this organicsolvent or solvent mixture. The polymerization takes place attemperatures between 100 and 160° C., preferably between 130 and 150° C.The polymerization initiators employed are preferably initiators whichform free radicals. The nature and amount of initiator are customarilychosen such that the supply of free radicals is largely constant duringthe feed phase at the polymerization temperature.

Examples of initiators which can be employed are dialkyl peroxides, suchas di-tert-butyl peroxide and dicumyl peroxide; hydroperoxides, such ascumene hydroperoxide and tert-butyl hydroperoxide; peresters, such astert-butyl perbenzoate, tert-butyl perpivalate, tert-butylper-3,5,5-trimethylhexanoate and tert-butyl per-2-ethylhexanoate; andbisazo compounds, such as azobisisobutyronitrile.

The polymerization conditions (reaction temperature, feed time of themonomer mixture, nature and amount of the organic solvents andpolymerization initiators, possible use of molecular weight regulators,such as mercaptans, thioglycolic esters and chlorinated hydrocarbons,for example) are selected such that the polyacrylate resins preferablyemployed have a number-average molecular weight of from 1500 to 10,000,preferably from 2500 to 5000 (determined by gel permeationchromatography using a polystyrene standard).

The acid number of the polyacrylate resins employed in accordance withthe invention can be established by the skilled worker by usingappropriate amounts of component (d). Similar comments apply to theestablishment of the hydroxyl number. It can be regulated by way of theamount of component (a) and (b) employed.

As component (a) use is made of 4-hydroxy-n-butyl acrylate,4-hydroxy-n-butyl methacrylate or a mixture of 4-hydroxy-n-butylacrylate and 4-hydroxy-n-butyl methacrylate. As component (a) it ispreferred to employ 4-hydroxy-n-butyl acrylate.

Further components suitable as polyacrylate component (a) are thehydroxy-functional compounds specified in European Patent Application EP0 767 185 and in the U.S. Pat. Nos. 5,480,943, 5,475,073 and 5,534,598.

Subject to the proviso that polymerization of component (b) alone givesa polyacrylate resin having a glass transition temperature of from 0 to+80, preferably from 0 to +60° C., the component (b) employed can inprinciple be any hydroxyl-containing ester of acrylic acid ormethacrylic acid, other than (a), or a mixture of such monomers.Examples are hydroxyalkyl esters of acrylic acid, such as hydroxyethylacrylate and hydroxypropyl acrylate, for example, and hydroxyalkylesters of methacrylic acid, such as hydroxyethyl methacrylate andhydroxypropyl methacrylate, for example, and also the products ofesterification of hydroxyalkyl (meth)acrylates with one or moremolecules of ε-caprolactone.

As component (c) it is possible in principle to employ any aliphatic orcycloaliphatic ester of methacrylic acid having at least 4 carbon atomsin the alcohol residue, other than (a) and (b), or a mixture of suchmonomers. Examples are aliphatic esters of methacrylic acid having 4 to20 carbon atoms in the alcohol residue, such as n-butyl, isobutyl,tert-butyl, 2-ethylhexyl, stearyl and lauryl methacrylate, for example,and cycloaliphatic esters of methacrylic acid, such as cyclohexylmethacrylate, for example. The composition of component (c) is selectedsuch that polymerization of component (c) alone gives a polymethacrylateresin having a glass transition temperature of from 0 to +80° C.,preferably from 0 to +60° C.

As component (d) it is possible in principle to employ any ethylenicallyunsaturated carboxylic acid or a mixture of ethylenically unsaturatedcarboxylic acids. As component (d) it is preferred to employ acrylicacid and/or methacrylic acid.

As component (e) it is possible in principle to employ any ethylenicallyunsaturated monomer other than (a), (b), (c) and (d), or a mixture ofsuch monomers. Examples of monomers which can be employed as component(e) are vinylaromatic hydrocarbons, such as styrene, α-alkylstyrene andvinyltoluene, amides of acrylic acid and methacrylic acid, such asmethacrylamide and acrylamide, for example; nitrites of methacrylic acidand acrylic acid; and vinyl ethers and vinyl esters. As component (e) itis preferred to employ vinylaromatic hydrocarbons, especially styrene.

The composition of component (e) is preferably selected such thatpolymerization of component (e) alone gives a polymer having a glasstransition temperature of from +70 to +120, preferably from +80 to +100°C.

The coating compositions employed in the process for producingscratch-resistant coatings comprise as crosslinker one or more free orblocked isocyanates and/or triazine-based components which crosslinkwith the hydroxyl groups of the binder to form ether and/or esterstructures. Where blocked isocyanates are present, the coatingcompositions of the invention are one-component (1C) clearcoats. Wherefree isocyanates are present, the coating compositions of the inventionare two-component (2C) clearcoats.

As crosslinker it is possible in principle to employ any polyisocyanatewhich can be employed in the coatings sector, or a mixture of suchpolyisocyanates, provided the cured coatings have the abovementionedviscoelastic properties. It is preferred, however, to employpolyisocyanates whose isocyanate groups are attached to aliphatic orcycloaliphatic radicals. Examples of such polyisocyanates arehexamethylene diisocyanate, isophorone diisocyanate,trimethylhex-methylene diisocyanate, dicyclohexylmethane diiso-cyanateand 1,3-bis(2-isocyanatoprop-2-yl)benzene (TMXDI), 1,3- and1,4-bis(isocyanatomethyl)cyclohexane, and adducts of thesepolyisocyanates with polyols, especially low molecular mass polyols,such as trimethylolpropane, for example, and polyisocyanates which arederived from these polyisocyanates and contain isocyanurate groupsand/or biuret groups. Polyisocyanates employed with particularpreference are hexamethylene diisocyanate and isophorone diisocyanate,polyisocyanates which are derived from these diisocyanates, containisocyanurate groups and/or biuret groups and preferably have more than 2isocyanate groups in the molecule, and also products of the reaction ofhexamethylene diisocyanate and isophorone diisocyanate or a mixture ofhexamethylene diisocyanate and isophorone diisocyanate with from 0.3 to0.5 equivalent of a low molecular mass polyol having a molecular weightof from 62 to 500, preferably from 104 to 204, in particular a triol,such as trimethylolpropane, for example.

For blocking the polyisocyanates it is possible in principle to employany blocking agent which can be employed for the blocking ofpolyisocyanates and has a sufficiently low deblocking temperature.Blocking agents of this kind are well known to the skilled worker andneed not be elucidated further here. It is preferred to employ blockedpolyisocyanates which contain both isocyanate groups blocked with ablocking agent (I) and isocyanate groups blocked with a blocking agent(II),

the blocking agent (I) being a dialkyl malonate or a mixture of dialkylmalonates,

the blocking agent (II) being a CH-acidic blocking agent other than (I),an oxime, or a mixture of these blocking agents, and

the ratio of equivalents between the isocyanate groups blocked with (I)and the isocyanate groups blocked with (II) being between 1.0:1.0 and9.0:1.0, preferably between 8.0:2.0 and 6.0:4.0 and, with particularpreference, between 7.5:2.5 and 6.5:3.5.

The blocked polyisocyanates employed with preference and theirpreparation are also described, for example, in DE-A-43 10 414, page 4line 56 to page 5 line 50, and the disclosure of this publication isincorporated herein by reference.

Dialkyl malonates or a mixture of dialkyl malonates are or is employedas the blocking agent (I). Examples of dialkyl malonates which can beemployed are dialkyl malonates having in each case 1 to 6 carbon atomsin the alkyl radicals, such as dimethyl malonate and diethyl malonate,for example, preference being given to the use of diethyl malonate.

The blocking agents used as (II) comprise blocking agents containingactive methylene groups, other than (I); oximes, and mixtures of theseblocking agents. Examples of the blocking agents (II) are methyl, ethyl,propyl, butyl, pentyl, hexyl, heptyl, octyl, nonyl, decyl or dodecylacetoacetate, acetone oxime, methyl ethyl ketoxime, acetylacetone,formaldoxime, acetaldoxime, benzophenoxime, acetoxime and diisobutylketoxime. As blocking agent (II) it is preferred to employ an alkylacetoacetate having 1 to 6 carbon atoms in the alkyl radical or amixture of such alkyl acetoacetates or a ketoxime or a mixture ofketoximes. Particular preference is given to the use of alkylacetoacetates or methyl ethyl ketoxime as blocking agent (II).

Further suitable blocking agents are dimethylpyrazole and/or triazoles.

The crosslinkers which react with the hydroxyl groups of the binder toform ether structures comprise amino resins. Amino resins are well knownto the skilled artisan and are offered by many companies as commercialproducts. They comprise condensates of aldehydes, especiallyformaldehyde, with, for example, urea, melamine, guanamine andbenzoguanamine. The amino resins contain alcohol groups, preferablymethylol groups, which in general are partially or, preferably, fullyetherified with alcohols. Use is made in particular ofmelamine-formaldehyde resins etherified with lower alcohols, especiallywith methanol or butanol. As crosslinkers, very particular preference isgiven to the use of melamine-formaldehyde resins which are etherifiedwith lower alcohols, especially with methanol and/or ethanol and/orbutanol, and which still contain on average from 0.1 to 0.25 hydrogenatoms attached to nitrogen atoms per triazine ring.

The triazine-based crosslinkers which react with the hydroxyl groups ofthe binder to form ester groups comprise transesterificationcrosslinkers, such as preferably tris(alkoxycarbonylamino)triazine orthe like, as also described, for example, in EP-A-604 922, and thedisclosure of this publication is incorporated herein by reference.

The coating compositions of the invention normally comprise binders andcrosslinkers in amounts such that the binder or binders is or arepresent in an amount of from 40 to 90, preferably from 50 to 75% byweight, and the crosslinker or crosslinkers in an amount of from 10 to60, preferably from 25 to 50% by weight, the weight percentages beingbased on binder+crosslinker=100% by weight.

The coating compositions of the invention are formulated on an aqueousbasis or, preferably, on a conventional basis, i.e., on the basis oforganic solvents. Examples of solvents suitable for the preparation ofthe conventional clearcoats are the solvents employed to prepare theacrylate resins.

The transparent coating compositions employed in accordance with theinvention contain only transparent pigments, if any. The coatingcompositions may, furthermore, also include further customary additives,such as light stabilizers, leveling assistants, etc., for example.

It is, however, also possible to prepare pigmented coating compositionswhich are not transparent. In order to achieve this, it is possible inprinciple to employ any suitable organic or inorganic pigments, ormixtures of such pigments, which are suitable for the production ofpigmented coating materials. Examples of pigments which can be employedare the following: azo pigments (e.g. Pigment Red 57:1, Pigment Yellow1, Pigment Yellow 13 and Pigment Red 7), phthalocyanine pigments (e.g.Pigment Blue 15:3 and phthalocyanine green), carbonyl pigments (e.g.Pigment Red 88, Pigment Red 177, Pigment Yellow 123, Pigment Violet 19,Pigment Yellow 24 and Pigment Orange 51 and 52), dioxazine pigments(e.g. Pigment Violet 23), titanium dioxide, carbon black, ion oxideblack (magnetite, triiron tetroxide), iron oxide red (hematite, diirontrioxide), iron oxide yellow (iron oxide hydroxide), iron oxide brown(mixed pigment of iron oxide red, iron oxide yellow and iron oxideblack), chromium oxide green (dichromium trioxide), nickel titaniumyellow, chromium titanium yellow, and cobalt blue. It is also possibleto employ effect pigments, examples being metal flake pigments,especially aluminum flake pigments, and pearl luster pigments.

In the preparation of pigmented coating materials which are nottransparent, binders, crosslinkers and the pigment or mixture ofpigments are generally employed in amounts such that the binder orbinders is or are present in an amount of from 39 to 90, preferably from45 to 75% by weight, the crosslinker or crosslinkers in an amount offrom 9 to 60, preferably from 20 to 50% by weight, and the pigment ormixture of pigments in an amount of from 1 to 40, preferably from 5 to15% by weight, the percentages by weight being based onbinder+crosslinker+pigment or mixture of pigments=100% by weight.

With the pigmented coating compositions which are not transparent it isalso possible to produce finishes, especially single-coat finishes,having very good properties.

The coating compositions of the invention can be applied to glass and awide variety of metal substrates, such as, for example, aluminum, steel,various iron alloys and the like. Preferably, they are employed as aclearcoat or topcoat in the field of automotive finishing (automotiveOEM finishing and—when free isocyanates are used—in the area ofautomotive refinishing as well). In addition to their application to awide variety of metals, the coating compositions can of course also beapplied to other substrates, such as, for example, wood, paper,plastics, mineral substrates or the like. They are, furthermore, alsosuitable for use in the field of the coating of packaging containers inthe field of the coating of films for the furniture industry and thelike.

Preferably, however, the coating compositions of the invention areemployed as topcoat in processes for producing a multicoat finish,especially in the field of OEM automotive finishing. The presentinvention therefore also relates to a process for producing multicoatfinishes in which

(1) a pigmented basecoat is applied to the substrate surface,

(2) a polymer film is formed from the basecoat,

(3) a transparent topcoat is applied atop the resultant basecoat film,and then

(4) the basecoat film and the topcoat film are cured together,

which comprises employing as topcoat a coating composition of theinvention.

In stage (1) of the process of the invention it is possible in principleto employ any pigmented basecoats suitable for producing two-coatfinishes. Such basecoats are well known to the skilled worker. It ispossible to employ both water-thinnable basecoats and basecoats based onorganic solvents. Suitable basecoats are described, for example, in U.S.Pat. No. 3,639,147, DE-A-33 33 072, DE-A-38 14 853, GB-A-2 012 191, U.S.Pat. No. 3,953,644, EP-A-260 447, DE-A-39 03 804, EP-A-320 552, DE-A-3628 124, U.S. Pat. No. 4,719,132, EP-A-297 576, EP-A-69 936, EP-A-89 497,EP-A-195 931, EP-A-228 003, EP-A-38 127 and DE-A-28 18 100, and thedisclosure of these publications are incorporated herein by reference.These patent documents also contain further information relating to thebasecoat/clearcoat technique in question.

In stage (2) of the process of the invention the basecoat film appliedin stage (1) is dried, i.e., in an evaporation phase, at least part ofthe organic solvents and/or of the water is removed from the basecoatfilm. The basecoat film is generally dried at temperatures ranging fromroom temperature up to 80° C.

Subsequently, the topcoat of the invention is applied and basecoat andtopcoat are cured together, customarily by heating at temperaturesranging from 120 to 155° C. for a period of from 20 to 40 minutes.Through a suitable choice of crosslinkers it is also possible to uselower stoving or heating temperatures, as customary in the field ofrefinishing and the coating of plastics, of below 100° C., preferablybelow 80° C.

The coatings produced using the coating compositions of the inventionare notable for a scratch resistance which is markedly improved relativeto conventional coatings.

The scratch resistance of the cured coatings can be assessed as followswith the aid, for example, of the BASF brush test as described in FIG. 2on page 28 of the article by P. Betz and A. Bartelt, Progress in OrganicCoatings, 22 (1993), pages 27-37, but modified in terms of the weightused (2000 g instead of the 280 g specified therein).

In this technique, the film surface is damaged using a weighted meshfabric. The mesh fabric and the film surface are wetted generously witha detergent solution. The test panel is moved forward and backward inreciprocal movements under the mesh fabric by means of a motor drive.

To produce the test panels, an electrodeposition coating material isapplied first of all in a film thickness of 18-22 μm, then a surfacer ina film thickness of 35-40 μm, then a black basecoat in a film thicknessof 20-25 μm and, finally, the test coating composition in a filmthickness of 40-45 μm, each of the films being cured. Followingapplication of the coating materials, the panels are stored at roomtemperature for at least 2 weeks before the test is conducted.

The test element is an eraser (4.5×2.0 cm, broad side perpendicular tothe direction of scratching) lined with nylon mesh fabric (No. 11, 31 μmmesh size, T_(g) 50° C.). The applied weight is 2000 g.

Prior to each test the mesh fabric is replaced, with the runningdirection of the fabric meshes parallel to the direction of scratching.Using a pipette, about 1 ml of a freshly stirred 0.25% strength Persilsolution is applied before the eraser. The rotational speed of the motoris set so that 80 double strokes are performed in a period of 80 s.After the test, the remaining washing liquid is rinsed off with cold tapwater and the test panel is blown dry using compressed air.

A measurement is made of the gloss in accordance with DIN 67530 beforeand after testing (direction of measurement perpendicular to thedirection of scratching).

Although there have been described what are presently considered to bethe preferred embodiments of the invention, it will be understood thatvariations and midifications may be made thereto within the scope of theappended claims.

What is claimed is:
 1. A process for assessing whether a coatingcomposition can provide a scratch-resistant coating comprising I.measuring, by dynamic mechanical thermoanalysis on a homogeneous freefilm having a film thickness of 40±10 μm, a storage modulus E′ and aloss factor tan δ of the coating c position, after the coatingcomposition has been cured, and II. labeling the coating composition asproviding a scratch-resistant coating when the storage modulus in arubber-elastic range is at least 10^(7.5) Pa and the loss factor tan δat 20° C. is at least 0.05; wherein the coating composition comprises A.at least one binder comprising at least one polyacrylate resin having ahydroxyl number of from 100 to 240, an acid number of from 0 to 35 and anumber-average molecular weight of from 1500 to 10,000, and B. at leastone crosslinker comprising at least one of a free isocyanate, a blockedisocyanate, and a triazine-based component wherein the triazine-basedcomponent crosslinks with hydroxyl groups of the binder to form at leastone of an ether and an ester structure.
 2. A process as claimed in claim1, wherein the coating composition after curing has at least one of astorage modulus E′ in the rubber-elastic range of at least 10^(7.6) Pa,and a loss factor tan δ at 20° C. of at least 0.07.
 3. A process asclaimed in claim 1, wherein the coating composition comprises as saidbinder at least one of: one or more polyacrylate resins having ahydroxyl number of more than 160 to 220; an acid number of from 0 to 25;and a number-average molecular weight of from 2500 to
 5000. 4. A processas claimed in claim 1, wherein the coating composition comprises as saidbinder one or more polyacrylate resins which have at least one of thefollowing characteristics: are prepared from not more than 15% byweight, based on the overall weight of the monomers employed to preparethe polyacrylate resin, of vinylaromatic hydrocarbons; has a glasstransition temperature of not more than +70° C.; and in which at least50%—of the OH groups are primary OH groups.
 5. A process as claimed inclaim 1, wherein the coating composition comprises as said binder one ormore polyacrylate resins, each of said polyacrylate resins comprisingthe reaction product of: (a) from 21 to 62% by weight of compoundsselected from the group consisting of 4-hydroxy-n-butyl acrylate,4-hydroxy-n-butyl methacrylate, and mixtures thereof, (b) from 0 to 36%by weight, of a non-(a) hydroxyl-containing ester of acrylic acid or ahydroxyl-containing ester of methacrylic acid or a mixture of suchmonomers, (c) from 28 to 75% by weight, compounds different from (a) and(b) comprising compounds selected from the group consisting of aliphaticand cycloaliphatic esters of methacrylic acid having at least 4 carbonatoms in the alcohol residue and mixtures of such monomers, (d) from 0to 3% by weight of an ethylenically unsaturated carboxylic acid or amixture of ethylenically unsaturated carboxylic acids, and (e) from 0 to20% by weight of ethylenically unsaturated monomers different from (a),(b), (c) and (d) and mixtures thereof, the sum of the weight fractionsof components (a), (b), (c), (d) and (e) always being 100% by weight andthe composition of component (c) being chosen such that polymerizationof component (c) alone gives a polymethacrylate resin having a glasstransition temperature of from 0 to +80° C.
 6. A process as claimed inclaim 5, wherein the coating composition comprises as said binder one ormore polyacrylate resins obtained using a component (e) selected suchthat polymerization of component (e) alone gives a polymer having aglass transition temperature of from +70 to +120° C.
 7. A process asclaimed in claim 1, wherein the coating composition comprises as saidcrosslinkers isocyanates which contain both isocyanates blocked with ablocking agent (I) and isocyanates blocked with a blocking agent (II),the blocking agent (I) being a dialkyl malonate or a mixture of dialkylmalonates, the blocking agent (II) being at least one of a CH-acidicblocking agent other than (I); and an oxime, and a ratio of equivalentsbetween the isocyanate groups blocked with (I) and the isocyanate groupsblocked with (II) being between 1.0:1.0 and 9.0:1.0.
 8. A process asclaimed in claim 1, wherein the coating composition comprises as saidcrosslinkers isocyanates having free isocyanate groups.
 9. A process asclaimed in claim 1, wherein the crosslinker consists of at least oneisocyanate and at least one tris(alkoxycarbonylamino)triazine.
 10. Aprocess as claimed in claim 1, wherein the coating composition aftercuring has at least one of a storage modulus E′ in the rubber-elasticrange of at least 10^(7.7) Pa, and a loss factor tan δ at 20° C. of atleast 0.07.
 11. A process as claimed in claim 1, wherein the coatingcomposition comprises as said binder at least one of: one or morepolyacrylate resins having a hydroxyl number of from 170 to 200, an acidnumber of from 0 to 25 and a number-average molecular weight of from2500 to
 5000. 12. A process as claimed in claim 1, wherein the coatingcomposition comprises as said binder one or more polyacrylate resinswhich have at least one of the following characteristics: are obtainedusing not more than 15% by weight, based on the overall weight of themonomers employed to prepare the polyacrylate resin, of vinylaromatichydrocarbons; has a glass transition temperature from −40 to +30° C.;and in which at least 50%—of the OH groups are primary OH groups.
 13. Aprocess as claimed in claim 5, wherein the polymerization of component(c) alone gives a polymethacrylate resin having a glass transitiontemperature of from 0 to +60° C.
 14. A process as claimed in claim 7,wherein the ratio of equivalents between the isocyanate groups blockedwith (1) and the isocyanate groups blocked with (II) is between 8.0:2.0and 6.0:4.0.
 15. A process as claimed in claim 7, wherein the ratio ofequivalents between the isocyanate groups blocked with (I) and theisocyanate groups blocked with (II) is between 7.5:2.5 and 6.5:3.5.